Conventionally, in a device used for outputting high-quality images, such as an electrophotographic copier, a printer, and particularly a device for outputting a color electrophotographic image, silicon rubber is often used as the material of a fixing member, so as to flexibly and closely contact the toner, and to attain heat resistance. However, such a heat-resistant rubber material has low heat conductivity, and thus acts as a heat-resistant layer when transferring heat from a heat source to a recording material. When producing color images, it is important to use a particularly soft rubber layer for improving the image quality. However, due to high heat capacity and high heat resistance, the startup time of the fixing device including such a fixing member is delayed. Furthermore, in the case of a high-speed device, the heat cannot be supplied quickly enough. Therefore, attempts have been made to increase the thermal conductivity with the use of filler, as disclosed in patent documents 1 and 2.    Patent Document 1: Japanese Laid-Open Patent Application No. 2006-133576    Patent Document 2: Japanese Laid-Open Patent Application No. 2005-292218
However, according to patent documents 1 and 2, the density of the fixing member cannot be decreased. By decreasing the density, the heat capacity can be decreased, which consequently speeds up the startup time. However, this cannot be accomplished in the conventional technology because the density cannot be decreased.
There are two reasons why the density of the fixing member cannot be decreased. The first reason is that as the expansion ratio is increased, the thermal conductivity may rapidly decrease. This happens when the thermal conductivity of the original substance does not change (for example, see “Netsu-bussei (thermal property) handbook”, published by Yoken-do, formula of Eucken on page 179 of C.2). The second reason is that as the expansion ratio is increased, the walls of the fixing member become relatively thin, and therefore the strength declines. The expansion ratio is expressed by (Vf+Vs)/Vs, where Vs is the volume of the fixing member when there are no air bubbles, and Vf is the volume of the air bubbles.
Due to the above two reasons, it has been difficult to develop a fixing device having low heat capacity (low density), high-thermal conductivity; and heat resistance, which is repeatedly deformed in a high-temperature environment.
Accordingly, there is a need for a fixing member having low heat capacity (low density), high-thermal conductivity, and a low rubber hardness, with which a fixing device of an image forming apparatus can start up at high speed, a manufacturing method of the fixing member, a rotational body for the fixing member, and a fixing device and an image forming apparatus including the fixing member.